Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC error code 71
Discharge temperature sensor error
Quick answer
On Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC, 71 is a fault. It means discharge temperature sensor error. A first thing to check: A failed discharge temperature thermistor, designated ThD.
- Applies to
- Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC
- Fujitsu GENERAL AIRSTAGE KMTC wall split codes, indoor and outdoor · Mini Split
- Type of code
- Fault
- Urgency: Needs attention soon
- First safe check
- Measure the sensor and compare it to 168.6 kOhm at 0 C, 62.6 kOhm at 20 C and 40.0 kOhm at 30 C.
What 71 means on this model
The thermistor on the compressor discharge line has failed, so the unit has stopped rather than run without the reading that protects the compressor. The documented test is a resistance check against the outdoor discharge thermistor curve: 168.6 kOhm at 0 C, 62.6 kOhm at 20 C, and 40.0 kOhm at 30 C. This is a sensor fault and it is not the same as code A1, which is a genuine discharge over-temperature condition, and keeping those two apart matters because one is a part and the other is a refrigerant or airflow problem.
Symptoms you will notice
- Code 71 and the system will not start
- The system will not recover after a defrost
- The unit runs erratically and will not hold setpoint
- The fault followed work on the outdoor unit
Common causes
The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:
- A failed discharge temperature thermistor, designated ThD
- A short or open in the wiring to the sensor
- A disturbed connector at the sensor
- A sensor whose resistance is out of range for the temperature it is reading
- A clip that has come loose from the discharge line
What you can safely check
These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.
- Measure the sensor and compare it to 168.6 kOhm at 0 C, 62.6 kOhm at 20 C and 40.0 kOhm at 30 C.
- Check that the sensor is still clipped to the discharge line and that the wire is not chafed against the hot pipe.
- Check the connector at the sensor.
Do not do these
- ×Do not treat this as the same fault as A1. A1 is a real over-temperature and this is a failed sensor, and they need different parts.
- ×Do not leave the sensor hanging off the pipe, because the control has no way to know the reading is not a pipe temperature.
- ×Do not work on the connector with the power switch on.
When to call a technician
Book a technician, do not keep resetting it
This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.
- The refrigerant circuit is sealed and legally requires licensed handling. Opening it without certification releases refrigerant and can put the whole system in violation of regulation, so a low-pressure or high-pressure fault on this equipment is a call-out, not a weekend job.
- If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.
A technician job.
What it costs to fix
Parts
Discharge temperature thermistor $30 to $90.
Labor
One to two hours to test and replace.
DIY or technician
A technician job.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- Fujitsu GENERAL AIRSTAGE KMTC wall split codes, indoor and outdoor
- Models in scope
- Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.